Electrical frequency-selective system



Dec. 16, 1941. .1.v ROBINSON 2,266,658

ELECTRICAL FREQUENCY-SELECTIVE SYSTEM Filed Sept. 28, 1938 2Sheets-Sheet 1 I f. Z1 Z0 In 511 1? 53 1 jTj Dec. 16,1941. J. ROBINSON2,266,653

ELECTRICAL FREQUENCY-SELECTIVE SYSTEM Filed Sept. 28, 1958 2Sheets-Sheet 2 I v a 1 Invg uter- MMYMM;

Patented Dec. 16, 1941 ELECTRICAL FREQUENCY-SELECTIV SYSTEM l JamesRobinson,- London, England Application September 28, 1938, Serial No.232,194 In Great Britain October 6, 1937 (or. 17s-44) 16 Claims.

This invention relates to electrical frequencyselective systems and isprimarily though not essentially concerned with such systems employingmechanically resonant selective devices or for example piezo-electricdevices.

It is one of the objects of the invention to provide a circuit affordinga generally uniform response over a given band of frequencies andabruptly reduced or negligible response to frequencies adjacent suchband, the frequency response curve being thus characterized by agenerally flat top with unusually sharp cut-off at each side. Variousmodifications of the circuit are contemplated affording variations ofthe response at different frequencies as hereinafter more fullydescribed.

According to a feature of the present invention there is provided anelectrical frequencyselective system comprising two conductive armsconstituting parallel paths for applied frequencies, a selective devicein each of these arms which selective devices have different resonantfrequencies, means combining the outputs of the two arms in the opposedsense and an additional conductive arm which is substantiallynonresonant in parallel with the others. By means of this system thereis provided a filter having a band-pass characteristic with a sharpcut-off at the limits of the band determined by the resonant frequenciesof the piezo-electric devices.

Further objects and features of the invention will be apparent from thefollowing description, taken in connection with the accompanyingdrawings in whichi Figure 1 is a diagram of a selective system accordingto the invention;

Figure 2 shows curves representing the performance of the systemillustrated in Figure 1;

Figure 3 shows a modification of Figure 1 employing retroaction;

Figure 4 shows another modification of Figure 1 employing aselectivedevice as an absorber;

Figure 5 shows a modification of Figure 1 employing highly selectivedevices coupled in series;

Figure 6 shows a curve representing the operation of the system shown inFigure 5;

Figure 7 shows a further modification of the system; 7 V

Figure 8 shows another selective system according to the invention;

Figure 9 is a curve representing the operation of the system shown inFigure 7;

Figure 10 is a curve showing the operation of the system shown in Figure8;

, II coupled thereto.

I Figure 11 shows another modification of the system shown in Figure 8;and

Figure 12 shows a modification of the system shown in Figure 11.

Like references indicate like parts in the several figures of thedrawings.

Referring to Figure 1, signals are applied across inductance ill from aninput inductance The inductance I0 is preferably tuned by a variablecondenser 12 and the ends of the inductance are connected respectivelyto one terminal of each of two piezo-electric devices l3 and M. Theother terminals of these piezo-electric devices are connected togetherat I! and to one side of a tuned circuit I5, the other side of the tunedcircuit being connected to a centre tapping I8 on the inductance It).With this arrangement, the inherent capacities of the piezo-electricdevices l3, M are normally equal so as to beelectrically balanced. Inparallel with the conductive arms of the circuit constituted by thepiezo-electric devices aforesaid there is the third conductive armconsisting of a variable condenser l6 connected in parallel with thepiezo-electric device l4 and thus unbalancing the system.

The piezo-electric devices are selected to have resonant frequenciescorresponding to the limit of the band of frequencies which the systemis required to pass and thus the resonant frequencies of the twopiezo-electric devices may, for example, be 5 kilocycles apart. Sincethese piezoelectric devices have their outputs combined in opposed phasethe total response for frequencies lying between the band is augmented,while the response for frequencies outside the band is diminished,because for frequencies within the band the outputs from the twopiezo-electric devices will be substantially in phase, Whereas forfrequencies above and below the band the outputs Will be opposed inphase and therefore reduced.

Referring to Figure 2, the piezo-electric device l4 has a resonantfrequency at f1 and the piezoelectric device l3 has a lower resonantfrequency at 1. Since their outputs are in opposed phase the totalresponse for frequencies lying within the band defined by their resonantfrequencies is augmented, while the response for other frequencies isdiminished, so that the overall characteristic for the system if the twopiezo-electric devices l3 and M are balanced is shown by the dotted line23 in Figure 2. The system is unbalanced by the condenser l6 providingan alternative path for applied signals. This alterna- 6 tive path beingcapacitative the phase of signals passed thereby is advanced through 90with respect to the signals passed by the piezo-electric devices atresonance so that at some higher frequency than the frequency f1, theresponse of the system will have a minimum or zero value, for instance,at f2 and at some frequency lower than the frequency f the response ofthe system will have a zero or minimum value as indicated at f3. At thesame time, the signals passed by the condenser |6 at frequencies betweenthe resonant frequencies of the piezo-electric devices will be insubstantial phase agreement with the signals of such frequencies passedby both these piezoelectric devices so that the response of the systemfor frequencies between the resonant frequencies of the piezo-electricdevices is raised. The characteristic of the system may thus have theform indicated by the broken line 24in Figure 3.

By increasing the capacity of the condenser l6 further to unbalance thesystem, the frequencies at which a minimum or zero response is obtainedmay be brought closer as indicated at f4 and f5 and the response forfrequencies between the resonant frequencies of the piezo-electricdevices may be made still more uniform so that the system has a responseof the form indicated for example by the full line curve 25 in Figure 2.

It will be seen that for frequencies more remote from the required bandthan those at which zero or minimum values occur, some response may beobtained, especially in the case where the system is considerablyunbalanced by the condenser I6 as shown by the curve 25. In order toovercome any disadvantage arising from this cause the circuit l 0, l2may be broadly tuned to the mean frequency of the required band so as toraise the response for frequencies within the required band and lowerthe response for other frequencies. The tuned circuit I5 is also tunedto an intermediate frequency of the required band in order further toassist in obtaining a uniform response throughout the required band anda minimum response outside the band.

Another manner of levelling the response for frequencies within therequired band consists in the appropriate selection of the relativeimpedances of the coupling of the system with associated apparatus. Thisis exemplified in Figure '1 by the connection of the output line to atapping l9 on'the inductance of the tuned circuit. The coupling is 50selected as to favour the transfer of signal frequencies within therequired band to associated apparatus.

A further method for correcting the shape of the response curve is shownin Figure 3. In this arrangement the filter of Figure 1 is provided withretroactive means for the purpose of raising the response of the systemfor frequencies within the required band and also lowering the responsefor frequencies outside the band. In Figure 3 the terminal I? isconnected to a control electrode of a thermionic valve 2'! and thecentre tapping IE on the inductance I6 is connected to the cathode ofthis valve, a leak resistance 28 being provided between the controlelectrode and the cathode. The anode circuit of the valve 2! includes acoil 29 retroactively coupled with the inductance l0 and phase adjustingmeans consisting of a variable resistance 30 and variable condenser 3!.The retroactive coupling is thus adjustable as regards phase to ensurethe proper phase relationship for building up the response within therequired band and this being the case it necessarily follows that theretroactive effect will be in the opposed sense for frequencies lying toeither side of the band.

A still further method for correcting the shape of the response curve ofthe filter of Figure 1 is shown in Figure 4 in which two additionalpiezo-electric devices 33 and 34 are connected between the junctionpoint I! and the centre tapping on the inductance III, that is to say,across the output of the system. These piezoelectric devices 33 and 34have substantially the same resonant frequencies as the piezo-electricdevices l3 and I4 and function as absorbers in the system compared withthe piezo-electric devices l3 and [4 operating as acceptors. A variableresistance 35 is preferably connected in series with each of thepiezo-electric devices 33 and 34.

By means of the piezo-electric devices 33 and 34'the two peaks in theoutput of the system due to the pieZo-electric devices l3 and I4 arereduced and may be controlled to have any desired value with respect totheresponse of the system for frequencies within the band by adjustmentof the resistances 35. The piezo-electric devices 33 and 34 operating asabsorbers reduce the response of the system at the resonant frequenciesof the pie'zo-electric devices [3 and I4 and immediately adjacentfrequencies, but have substantially no effect on other frequencies morewidely removed, with the result that the final output of the system moreclosely approaches a uniform response for frequencies within the bandwithout effecting the sharpness of the cut-off at the limits of theband.

It will be appreciated that the impedance matching method described withreference to Figure 1, the retroaction method of Figure 3 and theabsorption method of Figure 4 provide alternative methods of adjustingthe characteristic of the filter to any desired shape. They areparticularly useful when it is desired to limit the action of thecondenser IS in order to prevent a too great rise in the transmissionoutside the desired pass band. In some cases it' may be desirable to usetwo or more of these methods simultaneously.

It will also be appreciated that although the above describedarrangements all relate to band pass filters, the same configurationsmay be used to obtain band absorption filters. Thus if thepiezo-electric device l4 across which the condenser IB is connected hasthe lower frequency, the response above f1 and below will be increasedwhile the response between f1 and I will be decreased. The methods ofFigures 1, 3, and 4 may be similarly applied to obtain any desired shapeof characteristic.

Figure 5 shows a filter network in which two of the filters of Figure 1are connected in series to provide an improved response curve. Theoutput of the fist filter which comprises inductance l0, condenser l2,two piezo-electric devices l3 and I4, and-the condenser H3 in shunt withthe device I4 is developed across the inductance 4.4 which is coupledwith inductance 45. This inductance 45 together with condenser 52,piezo-electric devices 46 and 41 and the condenser 49 comprise thesecond filter, the output from which is developed across the inductance5E].

The piezo-electric device 46 has substantially the same resonantfrequency as the piezo-e1ec tric device l3, and the resonant frequenciesof the piezo-elec'tric devices l4 and 41 are also substantially thesame. By adjustment of the condenser 49 the second filter comprising thedevices.

piezo-electric device 46 and 41 is unbalanced to a different degree fromthat of the first filter comprising the piezo-electric devices [3 and M.For example the first filter may have a response as represented by curveof Figure 2 while the second filter may have a response as representedby curve 25 of Figure 2. In this manner the overall response of thesystem of Figure will be of the form indicated by the curve 5| in Figure6, in which there are minimum or zero values is and is at one side ofthe required band and at f2 and ii at the other side, thus furtherdecreasing the response for frequencies outside the required band. Theinductance 45 is tuned by a condenser 52 to the mean frequency of therequired band and the response of the system for frequencies within theband may be made still more uniform by employing any of the measures forthis purpose hereinbefore described.

Instead of employing piezo-electric devices 46 and 41 havingsubstantially the same resonant frequencies as the piezo-electricdevices l3 and I4 respectively, they may have different frequencies, forexample, such that one piezo-electric device of each series pair i3, 45and i4, 41 has a resonant frequency corresponding to the minimum or zerovalue of the response of the other.

In another form of frequency selective system according to thisinvention comprising two conductive arms, as aforesaid, a plurality ofselective devices is employed in each arm, these selective deviceshaving different resonant frequencies and being arranged alternately inthe two arms as regards the order of their resonant frequencies.

In Figure 7, there is employed an inductance ID to which signals areapplied by means of the input coil H coupled thereto. One end of theinductance ID is connected to terminals of two piezo-electric devices 54and 56 and the other end of the inductance i0 is connected to terminalsof two other piezo-electric devices 55 and 51 connected in parallel. Theother terminals of these piezo-electric devices are connected togetherat 58 and the output of the system is developed across an impedance 59connected between the junction point 58 and a centre tapping 60 on theinductance Ill. The piezo-electric devices 54 to 51 have resonantfrequencies of progressively increasing value so that the piezoelectricdevice 54 has the lowest frequency and the piezo-electric device 51 hasthe highest frequency. Additional piezo-electric devices may be employedif desired, and arranged in the same manner as regards their resonantfrequencies so as to be distributed over the required band. Athird'non-resonant conductive arm consisting of a condenser Si inparallel with the piezo-electric devices 54 and 55 may be employed ifdesired, in which case the signals passing the condenser will beadvanced in phase with respect to signals passing each of thepiezoelectric devices in resonance therewith.

It will be understood that due to the resonant characteristics of thepiezo-electric devices there will be a change of phase of the appliedsignals passed by these devices which is of the same sign through therange of frequencies between the resonant frequencies of thepiezo-electric crystals which are adjacent as regards frequency, andthat the change of phase will be of opposite sign for frequencies at theother sides of the resonant frequencies of these piezo-electric Thus, inemploying four selective devices, of which the resonant frequencies areindicated at f1, f2, is and f4 in Figure 9, the change of phase forfrequencies between ]2 and 1: will be of opposite sign to the change ofphase for frequencies between f1 and f2 and frequencies between f3 andit. The signals passed by the condenser Bl, which is connected inparallel with the piezo-electric devices 54 and 55, will be opposed inphase with regard to signals at frequencies between f1 and f2 andbetween f3 and f4. Thus, by appropriate adjustment of the condenser 6! azero response is obtained at a frequency between the frequencies f1 andf2 and at a frequency between the frequencies is and It. By increasingthe capacity of the condenser iii a zero response will be obtained attwo different frequencies between f1 and f2 and between is and f4 asshown in Figure 9. Thus, there is obtained a sharply defined band-passcharacteristic as indicated in Figure 9 with substantial zero responsefor a range of frequencies immediately each side of the band. The peaksat the frequencies f1 and f4 may be reduced by employing a broadly tunedcircuit tuned to the mean frequency of the band defined by thefrequencies f2 and is, for example by tuning the inductance ill by meansof the parallel condenser 12, by employing a tuned circuit as theimpedance 59, or by means of absorbers as described with reference toFigure 4. It will be appreciated that if a pluralityof piezo-electricdevices are provided in each parallel arm, this arrangement will enablea plurality of alternating band pass and band absorption channels to beobtained.

The system shown in Figure '7 may be modified to give a single band passcharacteristic by connecting the condenser 6| in parallel with thepiezo-electric devices 55 and 51 instead of the piezo-electric devices54 and 53, as shown in Figure 8. In this case a minimum response isobtained at a frequency lower than the frequency f1 and higher than thefrequency ii. The response for frequencies between T1 and f2 and betweenis and f4 are raised in this case and although the response forfrequencies between fz and is is reduced, by selecting thepiezo-electric devices to have resonant frequencies f2 and iscomparatively close together, the total response may be substantiallyuniform throughout the range ii to f4. This is illustrated in Figure 10of the drawing in which the response of the circuit of Figure8 withcondenser 5! omitted is shown in full lines, the effect of shunting thecondenser Bl about the piezo-electric devices 55 and 51 being indicatedby the dotted line curve.

In Figure 11 there is shown a modification of the system shown in Figure7 in which the piezoelectric devices 54 and 51 of lowest and highestfrequencies respectively are replaced by series tuned circuits Hi and H.In this case the final characteristic of the system will have the samegeneral form as that shown in Figure 9 except that the peaks atfrequencies f1 and f4 will be broader due to the use of the tunedcircuits.

It will be appreciated that instead of employing the two piezo-electricdevices to define the band, tuned circuits may be tuned for this purposeand the piezo-electric devices may have resonant frequencies, one higherand the other lower than the required band.

In another modification of the system shown in Figure 11, thepiezo-electric device 55 may have the same order of frequency as thetuned circuit 10 and similarly the piezo-electric device 56 may have thesame order of frequency as the tuned circuit H, in which case a bandpasscharacteristic is also obtained. A similar modification may be made inthe system shown in Figure 7.

Another modification of Figure 11 is shown in Figure 12 in which thepiezo-electric devices are joined at 80 and this junction point isconnected to the input electrode of a valve 82. The tuned circuits and Hare joined at an independent junction point 8! which is connected to theinput electrode of a valve 83. The output is derived from the anodes ofthe valves connected together, the cathodes being connected to thetapping 60 on the inductance H). In this case the condenser 6! providingthe additional conductive path of the system is connected in parallelwith the piezo-electric device 55 and therefore the tuning of thecircuits I0 and H is independent of the capacity of this condenser.

It is convenient in some circumstances to employ a signal response whichis higher for the two outer selective devices than for the inner onesand this result may be obtained by discriminate amplification or, forexample, in the case of piezo-electric devices, by supplementing thesepiezo-electric devices with other piezoelectric crystals of the sameorder of frequency in parallel therewith.

It will be understood that the piezo-electric devices hereinbeforereferred to may each consist of a single crystal or a plurality ofcrystals of substantially the same resonant frequncy connected in seriesor parallel.

Furthermore the use of one or more broadly tuned circuits tuned to anintermediate frequency of the required band, as described for examplewith reference to Figure l, or retroaction as described with referenceto Figure 3, or absorbers as described with reference to Figure 4, maybe applied to any other of the selective systems described herein.Alternatively or in addition one or more absorption circuits may be inthe system and tuned to a frequency or frequencies beyond the limits ofthe required band in order to still further reduce the total response ofsystem for frequencies outside the required band.

Whereas a tapped coil H) has been shown as a convenient means of feedingapplied frequencies along two conductive arms for their ultimatecombination in opposition, any other means for this purpose may beemployed.

It will also be understood that any of the circuits hereinbeforedescribed may be reversed as to their input and output in operation.

I claim:

1. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice in each of said arms, said selective devices having differentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, and means increasing theoutput of one of said arms for signals having a frequency lower than thefrequency of the associated selective device, and reducing the output ofsuch arm for signals having a frequency higher than the frequency of theassociated selective device, said means comprising, in parallel withsuch arm, an additional conductive arm which is substantiallynon-resonant.

2. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice in each of said arms, said selective devices having differentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, and means for altering therelative amplitude of the combined output for signals having frequenciesfalling within and without the said band, said means comprising anadditional conductive arm which is substantially non-resonant connectedin parallel with one of said first named arms.

3. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice in each of said arms, said selective devices having differentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, and means increasing thecombined output of said arms for signals having frequencies falling within the said band, said means comprising an additional conductive armwhich is substantially nonresonant connected in parallel with one ofsaid first named arms.

4. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice in each of said arms, said selective devices having differentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, and a non-resonantconductive device connected in parallel with that one of said selectivedevices which has th higher resonant frequency for increasing thecombined output of said arms for signals having frequencies fallingwithin the said band.

5. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice in each of said arms, said selective devices having differentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, and capacitative means inparallel with one of said devices for unbalancing the capacitative valueof the two arms to alter the relative amplitude of the output for signalfrequencies falling within and without the said band.

6. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice in each of said arms, said selective devices having differentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, means increasing the outputof one of said arms for signals having a frequency lower than thefrequency of the associated selective device, and reducing the output ofsuch arms for signals having a frequency higher than the frequency ofthe associated selective device, said means comprising, in parallel withsuch arm, an additional conductive arm which is substantiallynon-resonant, and at least 'one broadly tuned circuit which is tuned toa frequency intermediate the resonant frequency of said selectivedevices, said last named circuit being associated with said conductivearms and arranged in advance thereof in said system.

7. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice in each of said arms, said selective devices having difierentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, a third conductive armwhich is substantially nonresonant in parallel with one of said firstnamed arms, and phase changing means in said third conductive arm tochange the phase of frequencies passing said third conductive arm intoopposition with the frequencies passing said selective device andfalling outside of the said band of frequencies.

8. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, apiezoelectric device in each of said arms, said devices having differentresonant frequencies within a band of frequencies, means combining theoutputs of the two arms in phase opposition, and means increasing theoutput of one of said arms for signals having a frequency lower than thefrequency of the associated piezo-electric device and reducing theoutput of such arm for signals having a frequency higher than thefrequency of the associated piezo-electric device, said meanscomprising, in parallel with such arm, an additional conductive armwhich is substantially nonresonant,

9. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, apiezoelectric device in each of said arms, said piezoelectric deviceshaving different resonant frequencies within a band of frequencies,means combining the outputs of the two arms in phase opposition, andcapaoitative means in parallel with one of said devices for unbalancingthe capaoitative value of the two arms to alter the relative amplitudeof the output for signal frequencies fallin within and without the saidband.

10. An electrical frequency-selective system comprising two conductivearms constituting parallel paths for applied frequencies, apiezoelectric device in each of said arms, said piezoelectric deviceshaving different resonant frequencies within a band of frequencies,means combining the outputs of the two arms in phase opposition, and anon-resonant conductive device connected in parallel with that one ofsaid piezoelectric devices which has the higher resonant frequency forincreasing the combined output of said arms for signals havingfrequencies falling within the said band.

11. 'An electrical frequency selective system comprising two conductivearms constituting parallel paths for applied frequencies, a plurality ofpiezo-electric devices in each of said arms, said piezo-electric deviceshaving different resonant frequencies and being arranged in the two armsalternately as regards the order of their resonant frequencies, meansfor combining the outputs of the two arms in opposed phase, andnonresonant phase changing means connected in parallel with one of saidconductive arms to increase the output of the system in bands offrequencies defined by alternate pairs of the said piezo-electricdevices of adjacent frequency and to reduce the output of the systembetween the said bands such that a plurality of spaced band passchannels are provided.

12. An electrical frequency selective system comprising two conductivearms constituting parallel paths for applied frequencies, a plurality ofpiezo-electric devices in each of said arms, said piezo-electric deviceshaving different resonant frequencies and being arranged in the two armsalternately as regards the order of their resonant frequencies, meansfor combining the outputs of the two arms in opposed phase, andnon-resonant phase changing means connected in parallel with one of thesaid arms to reduce the output of the system for frequencies above thefrequency of the piezo-electric device having the highest resonantfrequency and below the frequency of the device having the lowestresonant frequency.

13. A frequency selective system comprising two conductive armsconstituting parallel paths for applied frequencies, a plurality ofselective devices in each of said arms which selective devices havedifferent resonant frequencies and are arranged in the said two armsalternately as regards the order of their resonant frequencies, theselective devices having the highest and the lowest resonant frequencybeing constituted by piezo-electric crystals, the other of said devicesbeing constituted by series tuned circuits, means for combining theoutputs of the two arms in opposed phase, and a non-resonant phasechanging device connected in parallel with one of said conductive arms.

14. A frequency selective system comprising two conductive armsconstituting parallel paths for applied frequencies, two selectivedevices consisting of a piezo-electric device and a series tuned circuitin each of said arms, said selective devices having different resonantfrequencies and being arranged in the said two arms alternately asregards the order of their resonant frequencies, the devices of highestand lowest frequency being constituted by the piezo-electric devices,means for combining the outputs of the two arms in opposed phase and acondenser, shunting that one of the said arms including the device ofhighest frequency, of such magnitude as to provide a substantiallyuniform response over a band of frequencies defined by the resonantfrequencies of the piezo-electric devices.

15. A band pass frequency selective system comprising two conductivearms constituting parallel paths for applied frequencies, a selectivedevice consisting of a piezo-electric device and a series tuned circuithaving the same resonant frequency in each of said arms, the saidselective devices having different resonant frequencies within the passband, means for combining the outputs of the two arms in opposed phaseand a condenser connected in shunt with one of said arms to increase theresponse of the system for frequencies within the pass band.

16. A frequency selective system comprising two conductive armsconstituting parallel paths for applied frequencies, a piezo-electricdevice in each of said arms, said piezo-electric devices havingdifferent resonant frequencies corresponding to the limits of a band offrequencies to be selected, means for combining the outputs of the twoarms in opposed phase, non-resonant phase changing means connected inshunt with the piezo-electric device of the higher resonant frequency,and a parallel circuit tuned to a frequency intermediate the limits ofthe said band of frequencies and connected in shunt with the output ofthe selective system.

JAMES ROBINSON.

